Polarizing beam splitter and methods of making same

ABSTRACT

Polarizing beam splitter is disclosed. The polarizing beam splitter includes a first polymeric prism, a second polymeric prism, a reflective polarizer that is disposed between and adhered to a hypotenuse of each of the first and second polymeric prisms, and a hardcoat that is disposed on each of the first and second polymeric prisms. The polarizing beam splitter includes an input major surface and an output major surface. At least one of the input and output major surfaces has a pencil hardness of at least 3H. The polarizing beam splitter has a low birefringence such that when polarized light having a first polarization state enters the optical element from the input major surface and travels through at least 2 mm of the polarizing beam splitter and exits the polarizing beam splitter from the output major surface, at least 95% of light exiting the polarizing beam splitter is polarized has the first polarization state.

FIELD

The present description relates to viewing devices. More specifically,the present description relates to head mounted viewing devices thatincorporate polarizing beam splitter plates that reflect imaged lighttoward a viewer with high effective resolution.

BACKGROUND

Illumination systems incorporating polarizing beam splitters (PBSs) areused to form images on viewing screens, such as projection displays. Atypical display image incorporates an illumination source that isarranged so that light rays from the illumination source reflect off ofan image-forming device (i.e., an imager) that contains the desiredimage to be projected. The system folds the light rays such that thelight rays from the illumination source and the light rays of theprojected image share the same physical space between a PBS and theimager. The PBS separates the incoming illumination light from thepolarization-rotated light from the imager. Due to new demands on PBSs,in part due to their new uses in applications such as, e.g.,three-dimensional projection and imaging, a number of new issues havearisen. The present application provides articles that address suchissues.

SUMMARY

In one aspect, the present description relates to a polarizationsubsystem. The polarization subsystem includes a first imager and apolarizing beam splitter. In some embodiments, the imager may be an LCOSimager. The polarizing beam splitter is made up in part of a reflectivepolarizer and receives imaged light from the imager. The reflectivepolarizer may be a multilayer optical film. In some embodiments, thereflective polarizer will have a surface roughness Ra of less than 45 nmor a surface roughness Rq of less than 80 nm. The polarizing beamsplitter reflects imaged light towards a viewer or screen with aneffective pixel resolution of less than 12 microns. In some embodiments,the polarizing beam splitter may reflect imaged light towards a vieweror screen with an effective pixel resolution of less than 9 microns, orless than 6 microns. The polarization subsystem may include a secondimager, where the polarizing beam splitter receives imaged light fromthe second imager at a different face from that where it receives lightfrom the first imager. The polarization subsystem may also include aprojection lens that projects light from the polarizing beam splittertowards a viewer or screen. In some cases, the polarization subsystemmay be part of a three-dimensional image projector.

In another aspect, the present description relates to a polarizing beamsplitter. The polarizing beam splitter includes a reflective polarizerthat is positioned between a first cover and a second cover. Thereflective polarizer may be a multilayer optical film. The polarizingbeam splitter is capable of reflecting imaged light towards a viewer orscreen with an effective pixel resolution of less than 12 microns, andpotentially less than 9 microns or less than 6 microns. The first and/orsecond covers of the polarizing beam splitter may be made, at least inpart, of glass or suitable optical plastic. The first and/or secondcovers may be attached to the reflective polarizer by a suitable opticaladhesive with additional processing, such as exposure to vacuum, toachieve the desired flatness of the multilayer optical film. Thereflective polarizer may have a surface roughness Ra of less than 45 nmor a surface roughness Rq of less than 80 nm.

In yet another aspect, the present description relates to a projectionsubsystem. The projection subsystem includes a light source, apolarizing beam splitter, at least a first imager, and potentially asecond imager. The polarizing beam splitter receives light from thelight source and includes a reflective polarizer made up of a multilayeroptical film. The first imager is positioned adjacent to the polarizingbeam splitter. The second imager is positioned adjacent to thepolarizing beam splitter on a different side of the polarizing beamsplitter than the first imager. Light from the light source is incidentupon the polarizing beam splitter and a first polarization of incidentlight is transmitted through the reflective polarizer while a secondpolarization of incident light orthogonal to the first polarizationstate is reflected by the reflective polarizer. Light of the secondpolarization travels from the polarizing beam splitter to the secondimager and is imaged and reflected back towards the polarizing beamsplitter. Light reflected from the second imager is transmitted throughthe polarizing beam splitter to an image plane. Light of the firstpolarization is transmitted through the polarizing beam splitter to thefirst imager and is imaged and reflected back towards the polarizingbeam splitter. Light reflected from the first imager is reflected at thepolarizing beam splitter towards an image plane with an effective pixelresolution of less than 12 microns. In at least some embodiments, lightreflected from the first imager is reflected at the polarizing beamsplitter towards an image plane with an effective resolution of lessthan 9 microns or less than 6 microns. The reflective polarizer may havea surface roughness Ra of less than 45 nm or a surface roughness Rq ofless than 80 nm. The light source of the projection subsystem may be anysuitable light source such as an arc lamp or an LED or LEDs.

In another aspect, the present description relates to a polarizationsubsystem. The polarization subsystem includes a first imager and apolarizing beam splitter. The polarizing beam splitter is made up inpart of a reflective polarizer and receives imaged light from theimager. The reflective polarizer may be a multilayer optical film. Thepolarizing beam splitter reflects imaged light towards a viewer orscreen. In some embodiments, the reflective polarizer has a surfaceroughness Ra of less than 45 nm or a surface roughness Rq of less than80 nm. In some embodiments, the reflective polarizer has a surfaceroughness Ra of less than 40 nm or a surface roughness Rq of less than70 nm. In some embodiments, the reflective polarizer has a surfaceroughness Ra of less than 35 nm or a surface roughness Rq of less than55 nm.

In another aspect, a polarization subsystem includes a first imager anda polarizing beam splitter plate adapted to receive imaged light fromthe imager. The polarizing beam splitter plate includes a firstsubstrate, a multilayer optical film reflective polarizer that isdisposed on the first substrate, a first outermost major surface, and anopposing second outermost major surface that makes an angle of less thanabout 20 degrees with the first outermost major surface. The polarizingbeam splitter plate reflects the received imaged light towards a vieweror a screen with the reflected imaged light having an effective pixelresolution of less than 12 microns.

In another aspect, a polarizing beam splitter plate includes a firstsubstrate, a second substrate, a multilayer optical film reflectivepolarizer that is disposed between and adhered to the first and secondsubstrates, a first outermost major surface, and an opposing secondoutermost major surface that makes an angle of less than about 20degrees with the first outermost major surface. The polarizing beamsplitter plate is adapted to reflect imaged light towards a viewer orscreen with the reflected imaged light having an effective pixelresolution of less than 12 microns.

In another aspect, a projection subsystem includes a light source, afirst imager that images light received from the light source, and apolarizing beam splitter plate that receives the imaged light from thefirst imager and includes a multilayer optical film reflectivepolarizer, a first outermost major surface, and an opposing secondoutermost major surface that makes an angle of less than about 20degrees with the first outermost major surface. The polarizing beamsplitter plate reflects the received imaged light towards an image planewith an effective pixel resolution of less than 12 microns.

In another aspect, a polarization subsystem includes a first imager, anda polarizing beam splitter plate that receives imaged light from theimager and includes a multilayer optical film reflective polarizer, afirst outermost major surface, and an opposing second outermost majorsurface that makes an angle of less than about 20 degrees with the firstoutermost major surface. The polarizing beam splitter plate reflects thereceived imaged light towards a viewer or screen. The multilayer opticalfilm reflective polarizer has a surface roughness Ra of less than 45 nmor a surface roughness Rq of less than 80 nm.

In another aspect, a method of producing a flat film includes the stepsof providing a multilayer optical film, providing a temporary flatsubstrate, releasably attaching a first surface of the multilayeroptical film to the temporary flat substrate, and providing a permanentsubstrate where the permanent substrate includes a first outermost majorsurface and an opposing second outermost major surface that makes anangle of less than about 20 degrees with the first outermost majorsurface. The method further includes the steps of attaching a secondsurface of the multilayer optical film to the permanent substrate, andremoving the multilayer optical film from the temporary flat substrate.

In another respect, a method of creating an optically flat polarizingbeam splitter plate, includes the steps of providing a multilayeroptical film reflective polarizer, applying a layer of pressuresensitive adhesive to a first surface of the multilayer optical film,applying a first substrate against the pressure sensitive adhesive layeron the side opposite the multilayer optical film where the firstsubstrate includes a first outermost major surface and an opposingsecond outermost major surface that makes an angle of less than about 20degrees with the first outermost major surface, and applying vacuum tothe pressure sensitive adhesive, the multilayer optical film, and thefirst substrate.

In another aspect, a viewing device includes a projector that projectinga first imaged light and a polarizing beam splitter plate that receivesthe projected first imaged light from the projector and reflects thereceived first imaged light for viewing by a viewer. The polarizing beamsplitter plate receives a second image and transmits the second imagefor viewing by the viewer. The polarizing beam splitter plate includes afirst substrate and a multilayer optical film reflective polarizer thatis adhered to the first substrate. The reflective polarizersubstantially reflects polarized light having a first polarization stateand substantially transmits polarized light having a second polarizationstate perpendicular to the first polarization state. The polarizing beamsplitter plate also includes a first outermost major surface and anopposing second outermost major surface that makes an angle of less thanabout 20 degrees with the first outermost major surface. The polarizingbeam splitter plate reflects the received first imaged light toward theviewer with the reflected first imaged light having an effective pixelresolution of less than 12 microns.

In another aspect, a polarizing beam splitter includes a first polymericprism, a second polymeric prism, a reflective polarizer that is disposedbetween and adhered to a hypotenuse of each of the first and secondpolymeric prisms, and a hardcoat that is disposed on each of the firstand second polymeric prisms. The polarizing beam splitter includes aninput major surface and an output major surface. At least one of theinput and output major surfaces has a pencil hardness of at least 3 H.The polarizing beam splitter has a low birefringence such that whenpolarized light having a first polarization state enters the opticalelement from the input major surface and travels through at least 2 mmof the polarizing beam splitter and exits the polarizing beam splitterfrom the output major surface, at least 95% of light exiting thepolarizing beam splitter is polarized has the first polarization state.

In another aspect, a method of producing an optical element includes thesteps of:

-   (a) providing an assembly that include a top polymeric sheet having    a top surface, a bottom polymeric sheet having a bottom surface, and    an optical film that is disposed between and adhered to the top and    bottom polymeric sheets;-   (b) modifying the top surface of the top polymeric sheet and the    bottom surface of the bottom polymeric sheet resulting in a top    structured surface having a plurality of top structures and a top    surface roughness, and a bottom structured surface having a    plurality of bottom structures and a bottom surface roughness;-   (c) forming a coated assembly by applying a top coating to the top    structured surface resulting in a top coated structured surface    having a plurality of top coated structures and a top coated surface    roughness less than the top surface roughness, and a bottom coating    to the bottom structured surface resulting in a bottom coated    structured surface having a plurality of bottom coated structures    and a bottom coated surface roughness less than the bottom surface    roughness; and (d) subdividing the coated assembly along a lateral    direction of the coated assembly into at least two discrete pieces    to form the optical element.

The following are a list items of the present disclosure:

Item 1 is a polarizing beam splitter comprising:

-   a first polymeric prism;-   a second polymeric prism;-   a reflective polarizer disposed between and adhered to a hypotenuse    of each of the first and second polymeric prisms, the reflective    polarizer substantially reflecting polarized light having a first    polarization state and substantially transmitting polarized light    having an opposite second polarization state;-   a hardcoat disposed on each of the first and second polymeric    prisms;-   an input major surface; and-   an output major surface, at least one of the input and output major    surfaces having a pencil hardness of at least 3 H, the polarizing    beam splitter having a low birefringence such that when polarized    light having a polarization state enters the optical element from    the input major surface and travels through at least 2 mm of the    polarizing beam splitter and exits the polarizing beam splitter from    the output major surface, at least 95% of light exiting the    polarizing beam splitter is polarized having the polarization state.

Item 2 is the polarizing beam splitter of item 1, wherein each of thefirst and second polymeric prisms is a right angle prism.

Item 3 is the polarizing beam splitter of item 1, wherein the first andsecond polymeric prisms form substantially a cube.

Item 4 is the polarizing beam splitter of item 1, wherein the reflectivepolarizer comprises a multilayer optical film reflective polarizer.

Item 5 is the polarizing beam splitter of item 1 having a maximumthickness of at least 5 mm.

Item 6 is the polarizing beam splitter of item 1, wherein each of theinput and output major surfaces has a pencil hardness of at least 3 H.

Item 7 is the polarizing beam splitter of item 1, wherein at least oneof the input and output major surfaces has a pencil hardness of at least4 H.

Item 8 is the polarizing beam splitter of item 1, wherein each of theinput and output major surfaces has a pencil hardness of at least 4 H.

Item 9 is the polarizing beam splitter of item 1, wherein at least oneof the input and output major surfaces has a pencil hardness of at least5 H.

Item 10 is the polarizing beam splitter of item 1, wherein each of theinput and output major surfaces has a pencil hardness of at least 5 H.

Item 11 is the polarizing beam splitter of item 1, wherein each majorsurface other than the hypotenuse of each of the first and secondpolymeric prisms has a pencil hardness of at least 3 H.

Item 12 is the polarizing beam splitter of item 1, wherein whenpolarized light having a polarization state enters the optical elementfrom the input major surface and travels through at least 2 mm of thepolarizing beam splitter and exits the polarizing beam splitter from theoutput major surface, at least 99% of light exiting the polarizing beamsplitter is polarized has the polarization state.

Item 13 is the polarizing beam splitter of item 1, wherein whenpolarized light having a polarization state enters the optical elementfrom the input major surface and travels through at least 2 mm of thepolarizing beam splitter and exits the polarizing beam splitter from theoutput major surface, at least 99.5% of light exiting the polarizingbeam splitter is polarized has the polarization state.

Item 14 is the polarizing beam splitter of item 1, wherein whenpolarized light having a polarization state enters the optical elementfrom the input major surface and travels through at least 5 mm of thepolarizing beam splitter and exits the polarizing beam splitter from theoutput major surface, at least 95% of light exiting the polarizing beamsplitter is polarized has the polarization state.

Item 15 is the polarizing beam splitter of item 1, wherein whenpolarized light having a polarization state enters the optical elementfrom the input major surface and travels through at least 5 mm of thepolarizing beam splitter and exits the polarizing beam splitter from theoutput major surface, at least 99% of light exiting the polarizing beamsplitter is polarized has the polarization state.

Item 16 is the polarizing beam splitter of item 1, wherein each of thefirst and second polymeric prisms comprises polymethylmethacrylate(PMMA).

Item 17 is the polarizing beam splitter of item 1, wherein the hardcoatcomprises an acrylate and/or a urethane.

Item 18 is the polarizing beam splitter of item 1, wherein the hardcoatcomprises silica and/or a ceramic material.

Item 19 is the polarizing beam splitter of item 1, wherein a maximumthickness of the hardcoat is less than about 15 microns.

Item 20 is the polarizing beam splitter of item 1, wherein a maximumthickness of the hardcoat is less than about 10 microns.

Item 21 is the polarizing beam splitter of item 1, wherein a maximumthickness of the hardcoat is less than about 5 microns.

Item 22 is the polarizing beam splitter of item 1, wherein a maximumthickness of the hardcoat is less than about 2 microns.

Item 23 is the polarizing beam splitter of item 1, wherein a differencebetween an index of refraction of each of the first and second polymericprisms and an index of refraction of the hardcoat is less than about0.05.

Item 24 is the polarizing beam splitter of item 1, wherein a differencebetween an index of refraction of each of the first and second polymericprisms and an index of refraction of the hardcoat is less than about0.025.

Item 25 is the polarizing beam splitter of item 1, wherein the inputmajor surface is parallel to the output major surface.

Item 26 is the polarizing beam splitter of item 1, wherein the inputmajor surface is perpendicular to the output major surface.

Item 27 is a method of producing an optical element, comprising thesteps of:

-   providing an assembly comprising a top polymeric sheet having a top    surface, a bottom polymeric sheet having a bottom surface, and an    optical film disposed between and adhered to the top and bottom    polymeric sheets;-   modifying the top surface of the top polymeric sheet and the bottom    surface of the bottom polymeric sheet resulting in a top structured    surface having a plurality of top structures and a top surface    roughness, and a bottom structured surface having a plurality of    bottom structures and a bottom surface roughness;-   forming a coated assembly by applying a top coating to the top    structured surface resulting in a top coated structured surface    having a plurality of top coated structures and a top coated surface    roughness less than the top surface roughness, and a bottom coating    to the bottom structured surface resulting in a bottom coated    structured surface having a plurality of bottom coated structures    and a bottom coated surface roughness less than the bottom surface    roughness;-   subdividing the coated assembly along a lateral direction of the    coated assembly into at least two discrete pieces to form the    optical element.

Item 28 is the method of item 27, wherein the optical element has aninput major surface, an output major surface, and a low birefringencesuch that when polarized light having a first polarization state entersthe optical element from the input major surface and travels through atleast 2 mm of the optical element and exits the optical element from theoutput major surface, at least 95% of light exiting the optical elementis polarized having the first polarization state.

Item 29 is the method of item 27, wherein the optical element has aninput major surface, an output major surface, and a low birefringencesuch that when polarized light having a first polarization state entersthe optical element from the input major surface and travels through atleast 2 mm of the optical element and exits the optical element from theoutput major surface, at least 97% of light exiting the optical elementis polarized having the first polarization state.

Item 30 is the method of item 27, wherein the optical element has aninput major surface, an output major surface, and a low birefringencesuch that when polarized light having a first polarization state entersthe optical element from the input major surface and travels through atleast 2 mm of the optical element and exits the optical element from theoutput major surface, at least 99% of light exiting the optical elementis polarized having the first polarization state.

Item 31 is the method of item 27, wherein the steps are carried outsequentially.

Item 32 is the method of item 27, wherein the step of providing theassembly comprises the steps of:

-   providing a temporary flat substrate;-   releasably attaching a first surface of the optical film to the    temporary flat substrate;-   adhering the top or bottom polymeric sheet to an opposing second    surface of the optical film; and-   removing the optical film from the temporary flat substrate.

Item 33 is the method of item 27, wherein the optical film is amultilayer optical film.

Item 34 is the method of item 27, wherein the optical film is amultilayer optical film reflective polarizer.

Item 35 is the method of item 27 further comprising the step ofannealing the top and bottom polymeric sheets resulting in the sheetshaving reduced optical birefringence.

Item 36 is the method of item 35, wherein the step of annealing thepolymeric sheets comprises heating the polymeric sheets.

Item 37 is the method of item 27, wherein the step of modifying the topsurface of the top polymeric sheet and the bottom surface of the bottompolymeric sheet comprises machining the top and bottom surfaces.

Item 38 is the method of item 27, wherein the step of modifying the topsurface of the top polymeric sheet and the bottom surface of the bottompolymeric sheet comprises fly-cutting the top and bottom surfaces.

Item 39 is the method of item 27, wherein the step of modifying the topsurface of the top polymeric sheet and the bottom surface of the bottompolymeric sheet comprises axial fly-cutting of the top and bottomsurfaces.

Item 40 is the method of item 27, wherein the step of modifying the topsurface of the top polymeric sheet and the bottom surface of the bottompolymeric sheet comprises radial fly-cutting of the top and bottomsurfaces.

Item 41 is the method of item 27, wherein the step of modifying the topsurface of the top polymeric sheet and the bottom surface of the bottompolymeric sheet comprises diamond end milling.

Item 42 is the method of item 27, wherein the step of modifying the topsurface of the top polymeric sheet and the bottom surface of the bottompolymeric sheet comprises diamond grinding.

Item 43 is the method of item 27, wherein each of a ratio of the topsurface roughness to the top coated surface roughness and a ratio of thebottom surface roughness to the bottom coated surface roughness is atleast 2.

Item 44 is the method of item 27, wherein each of a ratio of the topsurface roughness to the top coated surface roughness and a ratio of thebottom surface roughness to the bottom coated surface roughness is atleast 5.

Item 45 is the method of item 27, wherein the step of applying the topcoating to the top structured surface and a bottom coating to the bottomstructured surface comprises dipping the assembly into a coatingsolution.

Item 46 is the method of item 45 further comprising the steps ofremoving the assembly from the coating solution and drying the coatingon the assembly.

Item 47 is the method of item 27, wherein the step of applying the topcoating to the top structured surface and a bottom coating to the bottomstructured surface comprises spray coating the top and bottom structuredsurfaces.

Item 48 is the method of item 27, wherein the step of applying the topcoating to the top structured surface and a bottom coating to the bottomstructured surface comprises vacuum coating the top and bottomstructured surfaces.

Item 49 is the method of item 27, wherein the top and bottom coatingsare hard and/or anti-reflective coatings.

Item 50 is the method of item 27, wherein each of the top and bottomcoatings comprises multiple coatings.

Item 51 is the method of item 27, wherein a maximum thickness of each ofthe top and bottom polymeric sheets is at least 2 mm.

Item 52 is the method of item 27, wherein a maximum thickness of each ofthe top and bottom polymeric sheets is at least 5 mm.

Item 53 is the method of item 27, wherein the top structured surface hasa plurality of regularly arranged top structures and the bottomstructured surface has a plurality of regularly arranged bottomstructures.

Item 54 is the method of item 27, wherein each of the plurality of topstructures and the plurality of bottom structures comprises a pluralityof prisms.

Item 55 is the method of item 54, wherein each prism has a substantiallysquare hypotenuse.

Item 56 is the method of item 27, wherein there is a one to onecorrespondence between the top structures and the bottom structures.

Item 57 is the method of item 27, wherein the optical element is apolarizing beam splitter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a polarization conversion system according to the presentdescription.

FIG. 2 is a polarizing beam splitter according to the presentdescription.

FIG. 3 is a projection subsystem according to the present description.

FIG. 4 is a flowchart illustrating a method of making a flat multilayeroptical film for use in a PBS.

FIG. 5 illustrates a method for creating a polarizing beam splitterusing a multilayer optical film.

FIG. 6 is a schematic view of a polarization subsystem.

FIG. 7 is a schematic view of the outermost surfaces of a polarizingbeam splitter plate.

FIG. 8 is a schematic view of a reflective-type imaging system.

FIG. 9 is a schematic view of a transmissive-type imaging system.

FIG. 10 is a schematic view of a reflective-transmissive-type imagingsystem.

FIG. 11 is a schematic view of a viewing device.

FIG. 12 is a schematic view of a head mounted projection display.

FIGS. 13a-i illustrate a method of producing an optical element.

FIG. 14 is a schematic view of an optical element.

FIG. 15 is a schematic view of another optical element.

DETAILED DESCRIPTION

A high performance PBS is essential for creating a viable optical enginefor a projector that uses Liquid Crystal on Silicon (LCOS) imagers. Inaddition, a PBS may be required even for nominally unpolarized imagerssuch as DLP imagers when such imagers are required to handle polarizedlight. Typically, a PBS will transmit nominally p-polarized light andreflect nominally s-polarized light. A number of different types of PBSshave been used, including MacNeille type PBSs and wire grid polarizers.However, PBSs based on multilayer optical film have proven to be one ofthe most effective polarizing beam splitters for issues associated withlight handling in projection systems, including the ability toeffectively polarize over a range of wavelengths and angles of incidenceand with high efficiencies both in reflection and transmission. Suchmultilayer optical films are made by 3M Company, as described in U.S.Pat. No. 5,882,774 to Jonza et al., and U.S. Pat. No. 6,609,795 to Weberet al.

With the advent of a number of new imaging and projection applications,including, e.g., three-dimensional projection and imaging, newchallenges have arisen. Specifically, in at least some three-dimensionalimaging applications, it may be required that a PBS provide imaged lightthat has a high effective resolution (as defined below) not only whentransmitted through a reflective polarizing film, but also whenreflected by a reflective polarizing film. Unfortunately, polarizersbased on multilayer optical film, despite their other major advantages,may be difficult to formulate with the requisite flatness to reflectimaged light at high resolution. Rather, where such multilayer filmreflective polarizers are used to reflect imaged light, the reflectedimage may be distorted. However, the concerns of effectively polarizinga wide array of angles of incident light and wavelengths of incidentlight must still be addressed. It would therefore be highly desirable toprovide a polarizing beam splitter that has the benefits of a PBS thatcontains multilayer optical film, while also achieving heightenedeffective resolution for imaged light reflected off of the PBS towards aviewer or screen. The present description provides such a solution.

FIG. 1 provides an illustration of one polarization subsystem accordingto the present description. Polarization subsystem includes a firstimager 102. In a number of embodiments, such as that illustrated in FIG.1, the imager will be an appropriate reflective imager. Often, imagersused in projection systems are typically polarization-rotating,image-forming devices, such as liquid crystal display imagers, whichoperate by rotating the polarization of the light to produce an imagecorresponding to digital video signals. Such imagers, when used inprojection systems, typically rely on polarizers to separate light intoa pair of orthogonal polarization states (e.g., s-polarization andp-polarization). Two common imagers that may be used in the embodimentshown in FIG. 1 include a liquid crystal on silicon (LCOS) imager, ordigital light processing (DLP) imager. Those skilled in the art willrecognize that the DLP system will require some modification to theillumination geometry as well as an external means of rotating thepolarization (such as a retarder plate) in order to make use of the PBSconfiguration shown in FIG. 1. The polarization subsystem also includesa polarizing beam splitter (PBS) 104. Light 112 from a light source 110travels towards PBS 104. Within PBS 104 is a reflective polarizer 106.The reflective polarizer may be a multilayer optical film such as thoseavailable from 3M Company (St. Paul, Minn.) and described in, e.g., U.S.Pat. No. 5,882,774 to Jonza et al., and U.S. Pat. No. 6,609,795 to Weberet al., each of which is hereby incorporated by reference in itsentirety. When light 112 is incident upon film 106, one orthogonalpolarization state of the incident light, such as the p-polarized state,will be transmitted through the film and exit the PBS as light 120 thatis then incident on imager 102. The orthogonal polarization state of theincident light (in this case, s-polarized light), will be reflected byreflective polarizer 106 as a separate beam 118 in a differentdirection, here at right angles to beam 120. Unimaged light of a givenpolarization state 120 is incident upon imager 102. The light is thenimaged and reflected back towards PBS 104 and incorporated reflectivepolarizer 106. Where the imager 102 is an LCOS imager, and for thosepixels in an “on” state, light 114 is also converted to an orthogonalpolarization state. In this case, the p-polarized incident light, notyet imaged, is reflected as imaged light of s-polarization. When thes-polarized light is incident upon the polarizing beam splitter 104, andparticularly multilayer optical film reflective polarizer 106, the lightis reflected as s-polarized beam 116 towards a viewer or viewing screen130. Imager 102 can be any type imager that may be desirable in anapplication. For example, imager 102 can be an LCOS imager, an OLEDimager, a micro electro mechanical system (MEMS) imager, or a digitalmicro-mirror device (DMD) imager such as a DLP imager.

In a number of embodiments of the prior art, the imager may bepositioned, e.g., in the direction towards which beam 118 travels. Insuch an embodiment imaged light would be transmitted through thepolarizing beam splitter 104 rather than reflected in polarizing beamsplitter 104. Transmitting imaged light through the polarizing beamsplitter allows for less distortion of the image, and thus, highereffective resolution. However, as will be further explained, it may bedesirable in a number of embodiments to include an imager 102 aspositioned in FIG. 1. This may, for example, allow for overlappingimages of different polarizations. Despite the many benefits ofmultilayer optical film as a reflective polarizer, it has conventionallybeen difficult to achieve high effective resolution for imaged lightreflected off such films.

The Effective Resolution of the image or light produced by elements is auseful quantitative measurement because it helps predict what size pixelcan be reliably resolved. Most current imagers (LCOS and DLP) have apixel size range from about 12.5 μm down to around 5 μm. So in order tobe useful in a reflective imaging situation, the reflector must be ableto resolve down to at least about 12.5 μm, and ideally better. Thereforethe Effective Resolution of a PBS must be no more than about 12.5 μm,and preferably lower. This would be considered a high effectiveresolution.

Using techniques described in the specification, one may in fact providea multilayer optical film for use in a PBS 104 that can reflect imagedlight at very high resolution. In fact, looking to FIG. 1, imaged light116 may be reflected from the polarizing beam splitter 104 towards aviewer or viewing screen 130 with an effective pixel resolution of lessthan 12 microns. In fact, in some embodiments, the imaged light 116 maybe reflected from the polarizing beam splitter 104 towards a viewer orviewing screen 130 with an effective pixel resolution of less than 11microns, less than 10 microns, less than 9 microns, less than 8 microns,less than 7 microns, or potentially even less than 6 microns.

As discussed, in at least some embodiments, the polarization subsystem100 may include a second imager 108. Second imager 108 may generally beof the same type of imager as first imager 106, e.g., LCOS or DLP. Lightof one polarization state, such as s-polarized light, may be reflectedfrom PBS 104, and specifically from reflective polarizer 106 of the PBStowards the second imager. It may then be imaged and reflected backtowards PBS 104. Again, as with the first imager 104, light reflectedoff of second imager 108 is polarization converted, such that wheres-polarized unimaged light 118 is incident upon imager 108, p-polarizedimaged light 122 is redirected from the imager 108 back towards PBS 104.Whereas light 114 reflected from imager 102 is of a first polarizationstate (e.g., s-pol) and therefore reflects off of PBS 104 towards vieweror viewing screen 130, light reflected off of imager 108 (e.g. light122) is of a second polarization (e.g., p-pol.) and therefore istransmitted through PBS 104 towards viewer or viewing screen 130. As canbe seen from FIG. 1, the two imagers are located at different sides ofthe PBS 104, such that the PBS receives imaged light 114 from firstimager 102 at a first face 126 and receives imaged light 122 from thesecond imager 108 at a second face 124 different from the first face.

Once imaged light 116 and potentially light 122 exits PBS 104 it isdirected towards a viewer or viewing screen 130. In order to best directlight to the viewer and properly scale the image, light may be passedthrough a projection lens 128 or some sort of projection lens system.While only illustrated with a single element projection lens 128,polarization conversion system 100 may include additional imaging opticsas needed. For example, the projection lens 128 may in fact be aplurality of lenses, such as lens group 250 of commonly owned andassigned U.S. Pat. No. 7,901,083. Note that in the case that optionalimager 108 is not used, the input light 112 may be pre-polarized to havethe same polarization state as light beam 120. This can be accomplishedfor example, by the use of a polarization converting system (PCS), theaddition or a reflective or absorptive linear polarizer or other suchdevice for enhancing the polarization purity of the input light stream112. Such a technique may improve the overall efficiency of the system.

PBS 104 may include other elements besides reflective polarizer 106. Forexample, FIG. 1 illustrates a PBS 104 that also includes a first cover132 and a second cover 134. Reflective polarizer 106 is positionedbetween first cover 132 and second cover 134, such that it is bothprotected and properly positioned by the covers. The first cover 132 andsecond cover 134 may be made of any appropriate material known in theart, such as glass, plastic or potentially other appropriate materials.It should be understood that additional materials and constructions maybe applied to, e.g. the faces of the PBS or adjacent to andsubstantially coextensive with the reflective polarizer. Such othermaterials or constructions may include additional polarizers, dichroicfilters/reflectors, retarder plates, anti-reflection coatings, lensesmolded and/or bonded to the surface of the covers and the like.

Projection or polarization subsystems that emit light from differentimagers, wherein the imaged light is of different polarizations may beespecially useful as part of a three-dimensional image projector asdescribed for example in U.S. Pat. No. 7,690,796 (Bin et al.). Thedistinct advantage of using a PBS based two imager system is that notime sequencing or polarization sequencing is required. This means thatboth imagers are operating at all times, effectively doubling the lightoutput of the projector. As discussed, it is highly important that thereflective polarizer 106 be flat, such that the imaged light 116reflected off of the polarizer is not distorted and has high effectiveresolution. Flatness can be quantified by the standard roughnessparameters Ra (the average of the absolute value of the verticaldeviation of the surface from the mean), Rq (the root mean squaredaverage of the vertical deviation of the surface from the mean), and Rz(the average distance between the highest peak and lowest valley in eachsampling length). Specifically, the reflective polarizer preferably hasa surface roughness Ra of less than 45 nm or a surface roughness Rq ofless than 80 nm, and more preferably has a surface roughness Ra of lessthan 40 nm or a surface roughness Rq of less than 70 nm, and even morepreferably has a surface roughness Ra of less than 35 nm or a surfaceroughness Rq of less than 55 nm. One exemplary method of measuring thesurface roughness or flatness of the film is provided in the Examplessection below.

In another aspect, the present description relates to a polarizing beamsplitter. One such polarizing beam splitter 200 is illustrated in FIG.2. Polarizing beam splitter 200 includes a reflective polarizer 206 thatis positioned between a first cover 232 and a second cover 234. As withreflective polarizer 106 of FIG. 1, the reflective polarizer 206 of FIG.2 is a multilayer optical film such as those described above. Thepolarizing beam splitter 200 is capable of reflecting imaged light 216towards a viewer or surface 230. The effective pixel resolution of theimaged light 216 that is directed towards the viewer or surface is lessthan 12 microns, and possibly less than 11 microns, less than 10microns, less than 9 microns, less than 8 microns, less than 7 microns,or potentially even less than 6 microns.

As with the covers of FIG. 1, first cover 232 and second cover 234 ofPBS 200 may be made of any number of appropriate materials used in thefield, such as glass or optical plastics, among others. In addition, thefirst cover 232, and second cover 234 may each be attached to reflectivepolarizer 206 by a number of different means. For instance, in oneembodiment, the first cover 232 may be attached to the reflectivepolarizer 206 using a pressure sensitive adhesive layer 240. A suitablepressure sensitive adhesive is 3M™ Optically Clear Adhesive 8141(available from 3M Company, St. Paul, Minn.). Similarly, the secondcover 234 may be attached to the reflective polarizer using a pressuresensitive adhesive layer 242. In other embodiments, the first and secondcover may be attached to reflective polarizer 206 using differentadhesive types for layer 240 and 242. For example, layers 240 and 242may be made up of a curable optical adhesive. Suitable optical adhesivesmay include optical adhesives from Norland Products Inc. (Cranbury,N.J.), such as NOA73, NOA75, NOA76 or NOA78, the optical adhesivesdescribed in commonly owned and assigned U.S. Patent Publication No.2006/0221447 (to DiZio et al.) and commonly owned and assigned U.S.Patent Publication No. 2008/0079903 (to DiZio et al.), each of which ishereby incorporated by reference. UV curable adhesives may also be used.It should be understood that additional materials and constructions maybe applied to, e.g. the faces of the PBS or adjacent to andsubstantially coextensive with the reflective polarizer. Such othermaterials or constructions may include additional polarizers, dichroicfilters/reflectors, retarder plates, anti-reflection coatings, and thelike. As with the PBS described in FIG. 1, the reflective polarizer 206of FIG. 2 must be very flat to most effectively reflect imaged light 216without distorting it. The reflective polarizer may have a surfaceroughness Ra of less than 45 nm or a surface roughness Rq of less than80 nm. With typical application procedures of pressure sensitiveadhesives such as described in U.S. Pat. No. 7,234,816 B2 (Bruzzone etal.) the required surface flatness of the reflective polarizer is notachieved. It has been discovered that certain types of postprocessing,allow the required surface flatness to be achieved.

In yet another aspect, the present description relates to a projectionsubsystem. One such projection subsystem is illustrated in FIG. 3.Projection subsystem 300 includes a light source 310. Light source 310may be any number of appropriate light sources commonly used inprojection systems. For example, the light source 310 may be asolid-state emitter such as a laser or light emitting diode (LED)emitting light of a specific color such as red, green, or blue light.The light source 310 may also include a phosphor or other lightconverting material that absorbs light from the emissive source andre-emits light at other (generally longer) wavelengths. Suitablephosphors include well known inorganic phosphors such as Ce-doped YAG,strontium thiogallate, and doped silicate and SiAlON-type materials.Other light converting materials include III-V and II-VI semiconductors,quantum dots, and organic fluorescent dyes. Alternatively, the lightsource may be made up of a plurality of light sources, such as a red, agreen and a blue LED, where such LEDs may be activated together orsequentially. Light source 310 may also be a laser light source, orpotentially a traditional UHP lamp. It is to be understood thatancillary components such as color wheels, dichroic filters orreflectors and the like may additionally comprise light source 310.

The projection subsystem 300 further includes a polarizing beam splitter304. Polarizing beam splitter 304 is positioned such that it receiveslight 312 from the light source. This incident light 312 may generallybe made up in part of two orthogonal polarization states, e.g., parts-polarized light, and part p-polarized light. Within the polarizingbeam splitter is a reflective polarizer 306, again in this case amultilayer optical film such as those described with respect toreflective polarizer 106. Light 312 is incident upon reflectivepolarizer 306 and light of one first polarization, e.g., p-polarizedlight is transmitted through as light 320 while light of a secondorthogonal polarization, e.g. s-polarized light, is reflected as light318.

Light of the first polarization 320 that is transmitted through thereflective polarizer 306 travels towards a first imager 302 that ispositioned adjacent to the PBS 304. Light is imaged and reflected at thefirst imager 302 back towards PBS 304 with the polarization of the lightconverted. The converted imaged light 314 is then reflected at the PBS304 as light 316 towards an image plane 350. The light 316 is reflectedoff of the reflective polarizer 306 of the PBS and reaches image plane350 with an effective resolution of less than 12 microns, and possiblyless than 11 microns, less than 10 microns, less than 9 microns, lessthan 8 microns, less than 7 microns, or potentially even less than 6microns. The reflective polarizer 306 typically has a surface roughnessRa of less than 45 nm or a surface roughness Rq of less than 80 nm.

Light of the second polarization (e.g. s-polarized) light that isreflected initially by the reflective polarizer of PBS 304 travels aslight 318 towards a second imager 308. Second imager 308 is alsopositioned adjacent the PBS 304, as with first imager 302, but secondimager is positioned on a different side of the PBS. The incident light318 is imaged and reflected back towards PBS 304. Upon reflection fromthe imager, the polarization of this light is rotated as well by 90degrees (e.g. from s-polarized light to p-polarized light). The imagedlight 322 is transmitted through the PBS 304 to the image plane 350. Thefirst imager 302 and second imager 308 may be any appropriate type ofreflective imager, such as those described above with respect toelements 102 and 108 of FIG. 1.

As discussed, in order to achieve high effective resolution for imagedlight reflected off of the PBS herein, the reflective polarizer of thePBS must be exceptionally optically flat. The present description nowprovides methods of producing an optically flat reflective polarizerthat is a multilayer optical film and/or methods of producing anoptically flat polarizing beam splitter.

One such method is illustrated in the flowchart of FIG. 4. The methodbegins with providing a multilayer optical film 410, and providing aflat substrate 420. The multilayer optical film 410 may be similar tothe multilayer optical films described with respect to the articlesabove. The flat substrate may be any number of appropriate materials,such as acrylic, glass or other appropriate plastics. Most importantly,the substrate 420 must possess at least the same degree of opticalflatness as is required in the polarizing beam splitter and must allow awetting solution to spread over its surface. Therefore, other plastics,inorganic glasses, ceramics, semiconductors, metals or polymers may beappropriate materials. Additionally it is useful for the substrate to beslightly flexible.

In the next step, the surface 425 of the flat substrate is releasablyattached to a first surface of the multilayer optical film. In at leastone embodiment, in order to create a releasable attachment, either thesurface 425 of the flat substrate, or a first surface of the multilayeroptical film, or both is wetted with a wetting agent, resulting in athin layer of solution 430. A suitable wetting agent should have asurface energy that is sufficiently low that it will wet out thesubstrate or the film and a vapor pressure that is sufficiently highthat it can evaporate at room temperature. In some embodiments,isopropyl alcohol is used as the wetting agent. In at least someembodiments the wetting agent will be an aqueous solution that containsat least a small amount of surfactant (e.g. less than 1% by volume). Thesurfactant may be common commercially available industrial wettingagents, or even household materials such as dishwashing detergent. Otherembodiments may be aqueous mixtures of compounds that leave no residueupon evaporation such as ammonia, vinegar, or alcohol. The wetting agentmay be applied by a number of appropriate methods including spraying,e.g., from a spray bottle. In the next step, the multilayer optical filmis applied to the surface of the substrate 425 such that the solution430 is sandwiched between the film and substrate. Typically the wettingagent is applied to the contacting surface of the multilayer opticalfilm also. A pressure applying instrument 435, such as a squeegee isthen drawn across the top of multilayer optical film 410 closelyflattening optical film 410 to the surface 425 of substrate 420, andleaving only a thin, fairly uniform layer of solution 430 separating thetwo. In at least some embodiments, a protective layer may first beapplied to the multilayer optical film on the side opposite the surface440 that is applied to the substrate 420. At this point, theconstruction is left to allow the solution 430 to evaporate. Thesqueegeeing process pushes residual water past the edges of themultilayer optical film such that only a small amount remains. Next, themultilayer optical film, flat substrate, and wetting agent are allowedto dry. With time, all of the volatile components of the wettingsolution evaporate either through layers 410 or 420 or by wicking alongthe space between layers 410 and 420 to the edges of layer 410 whereevaporation can occur. As this process occurs, the multilayer opticalfilm 410 is drawn closer and closer to substrate 420 until layer 410closely conforms to the surface 425. The result is shown in the nextstep of FIG. 4, as the drying closely draws the film 410 to substrate420 and effectively flattens the bottom surface 440 of the multilayeroptical film. Once this flatness has been achieved, the multilayeroptical film 410 remains stably flat but releasably attached to thesubstrate. At this point a permanent substrate may be adhered to theexposed surface of the film 410.

FIG. 5 illustrates further steps that may be taken in providing a finalconstruction of a polarizing beam splitter. For example, an adhesive 550may be applied on the flattened surface 450 of film 410. The adhesivemay be any appropriate adhesive that does not adversely affect theoptical or mechanical performance of the PBS. In some embodiments, theadhesive may be a curable optical adhesive, such as NOA73, NOA75, NOA76or NOA78 from Norland Products Inc. (Cranbury, N.J.). In otherembodiments, optical epoxies may be used. In some embodiments, theadhesive may be a pressure sensitive adhesive. Next, one may provide apermanent second substrate. In one embodiment, the permanent secondsubstrate may be a prism. As shown in FIG. 5, Prism 560 is appliedagainst the adhesive 550 and the construction is cured if appropriate.The film 410 may now be removed from the substrate 420. In at least oneembodiment, the film 410 is peeled away from substrate 420, typically byflexing substrate 420 slightly to allow the film 410 to release fromsubstrate 420. For cured adhesives such as UV adhesives or epoxies thenewly exposed bottom surface of the film 440 retains the flatness of thesubstrate 420. For pressure sensitive adhesives, the bottom surface ofthe film 440 may retain the flatness of the substrate 420 or may requireadditional processing to maintain the flatness. Once the flat filmsurface 440 has been achieved a second layer of adhesive 570 may beapplied to the bottom surface of the film 440 and a second prism orother permanent substrate 580 may be applied to the adhesive. Again theconstruction may be cured as needed, resulting in a complete polarizingbeam splitter.

Another method of making an optically flat polarizing beam splitterincludes the use, specifically, of pressure sensitive adhesives. Withappropriate techniques, the multilayer optical film may be made toconform closely to the flat surface of the prism. The following stepsmay be included. First, a multilayer optical film is provided. Themultilayer optical film will act as a reflective polarizer. This may besimilar to reflective polarizer optical film 410 of FIG. 5 with theexception that the surface 440 may not be substantially flattenedalready through the steps shown in FIG. 4. A layer of pressure sensitiveadhesive (here corresponding to adhesive layer 550) may be applied tothe first surface 440 of the multilayer optical film. Next a prism 560may be applied against the pressure sensitive adhesive layer adhesivelayer on the side opposite the multilayer optical film 410. The methodmay also include applying a second layer of adhesive (e.g. layer 570) ona second surface 575 of the film opposite first surface 440. A secondprism 580 may then be applied to the opposite side of layer 570 fromfilm 410. The present method provides an improvement over this methodthat further enhances the flatness of the reflective polarizer/prisminterface, such that imaged reflection off of the PBS has enhancedresolution. After pressure sensitive adhesive 550 is applied between theprism 560 and multilayer optical film 410, the construction is subjectedto vacuum. This may occur, for example, by placing the construction in avacuum chamber equipped with a conventional vacuum pump. The vacuumchamber may be lowered to a given pressure, and the sample may be heldat that pressure for a given amount of time, e.g., 5-20 minutes. Whenair is re-introduced to the vacuum chamber, the air pressure pushes theprism 560 and multilayer optical film 410 together. Where a secondadhesive layer and second prism are also applied, the subjection tovacuum in the chamber may optionally be repeated for the secondinterface (e.g. at layer 570). Applying vacuum to a prism/MOF assemblyresults in a PBS that provides heightened effective resolution whenimaged light is reflected off of the PBS. In place of or in conjunctionwith the vacuum treatment, a thermal/pressure treatment may also beused. It may be advantageous to conduct the processing more than onetime.

EXAMPLES

The following list of materials and their source is referred tothroughout the Examples. If not otherwise specified, materials areavailable from Aldrich Chemical (Milwaukee, Wis.). Multilayer OpticalFilms (MOFs) were generally prepared according to methods described in,for example, U.S. Pat. No. 6,179,948 (Merrill et al); U.S. Pat. No.6,827,886 (Neavin et al); 2006/0084780 (Hebrink et al); 2006/0226561(Merrill et al.); and 2007/0047080 (Stover et al.).

Roughness Measurement Method

Prisms were placed on modeling clay and leveled using a plunger leveler.Topographic maps were measured with a Wyko® 9800 optical interferometer(available from Veeco Metrology, Inc., Tucson, Ariz.), with a 10×objective and 0.5× field lens and with the following settings: VSIdetection; 4 mm×4 mm scan area stitched using 6 rows and 5 columns ofindividual maps, 2196×2196 pixels with a sampling of 1.82 μm; tilt andsphere correction used; 30-60 microns back scan length with 60-100forward scan length; with the modulation detection threshold 2%.Autoscan detection was enabled at 95% with 10 μm post scan length (thisshort post scan length avoided subsurface reflections in the datacollection).

A 4 mm×4 mm area in the central region of the hypotenuse-face of eachprism was measured. Specifically, the topography of each region wasmeasured, plotted, and the roughness parameters Ra, Rq and Rz werecalculated. One measurement area was obtained per prism. Three prismsamples were measured in each case and the mean and standard deviationof the roughness parameters were determined.

Example 1 Wet Application Method

A reflective polarizing multilayer optical film (MOF) was releasablydisposed onto an optically flat substrate in the following manner. Firsta wetting solution comprising approximately 0.5% mild dishwashingdetergent in water was placed into a spray bottle. A sheet ofapproximately 6 mm high-gloss acrylic was obtained and the protectivelayer removed from one side in a clean hood. The exposed acrylic surfacewas sprayed with the wetting solution so that the entire surface waswet. Separately a piece of MOF was obtained and one of its skin layerswas removed in a clean hood. The exposed surface of the MOF was sprayedwith the wetting solution, and the wet surface of the MOF was contactedwith the wet surface of the acrylic sheet. A heavy release liner wasapplied to the surface of the MOF to prevent damage to the MOF, and a3M™ PA-1 applicator (available from 3M Company, St. Paul, Minn.) wasused to squeegee the MOF down to the surface of the acrylic. Thisresulted in most of the wetting solution being expelled from between thetwo wetted surfaces. After this was done the second skin layer from theMOF was removed. Inspection of the applied MOF showed that the MOFsurface was much more irregular than the surface of the acrylic. Uponinspection again after 24 hours, the MOF surface was observed to becomparable in flatness to the acrylic sheet. This observed flatteningover time is consistent with residual wetting solution evaporating frombetween the two surfaces allowing the MOF to conform closely to thesurface of the acrylic. Even though the MOF conformed closely and stablyto the surface of the acrylic, it could be easily removed by peeling theMOF from the surface of the acrylic.

An imaging PBS was prepared by placing a small amount of Norland OpticalAdhesive 73 (available from Norland Products, Cranbury, N.J.) onto thesurface of the MOF. The hypotenuse of a 10 mm 45° BK7 polished glassprism was slowly placed into contact with the adhesive so that nobubbles were entrained in the adhesive. The amount of adhesive waschosen so that when the prism was placed on to the adhesive, there wassufficient adhesive to flow out to the edges of the prism, but not somuch adhesive to cause substantial overflow of the adhesive beyond theperimeter of the prism. The result was that the prism was substantiallyparallel to the surface of the MOF and separated by a layer of adhesiveof approximately uniform thickness.

A UV curing lamp was used to cure the adhesive layer through the prism.After curing, a section of the MOF that was larger than the prism andthat contained the prism was peeled away from the acrylic substrate.Removal was facilitated by bending the acrylic plate, thereby allowingthe rigid prism and MOF composite to separate more easily from theacrylic plate. Inspection of the prism/MOF composite showed that the MOFretained its flatness despite being removed from the acrylic plate.

The roughness parameters of the MOF were then measured as describedunder “Roughness Measurement Method” and are reported in the followingtable.

average stdev Ra (nm) 34 12 Rq (nm) 51 30 Rz (μm) 6.7 8.5A small amount of the Norland optical adhesive was applied to the MOFsurface on the prism/MOF composite. A second 10 mm 45° prism wasprocured and its hypotenuse placed in contact with the adhesive. Thesecond prism was aligned such that its principal and secondary axes weresubstantially parallel to those of the first prism, and the twohypotenuse surfaces were substantially coextensive. A UV curing lamp wasused to cure the adhesive layer so that the second 45° prism was bondedto the prism/MOF composite. The resulting configuration was a polarizingbeam splitter.

Example 2 PSA Method Using Heat and Pressure

An adhesive construction was formed by taking a sample of 3M™ OpticallyClear Adhesive 8141 (available from 3M Company, St. Paul, Minn.) andlaminating it to a reflective polarizing MOF using a roll laminationprocess. A piece of this adhesive construction was adhered to thehypotenuse of a glass prism similar to that used in Example 1. Theresulting MOF/prism composite was placed into an autoclave oven andprocessed at 60° C. and 550 kPa (80 psi) for two hours. The sample wasremoved and a small quantity of thermally curable optical epoxy wasapplied to the MOF surface of the MOF/prism composite. The prisms werealigned as in Example 1. The sample was then returned to the oven andagain processed at 60° C. and 550 kPa (80 psi), this time for 24 hours.The resulting configuration was a polarizing beam splitter.

Example 2A Roughness Resulting from PSA Method Using Heat and Pressure

The roughness of MOF produced using the method of Example 2 wasdetermined as follows. A piece of MOF measuring 17 mm×17 mm waslaminated using a hand roller to a glass cube having a width of 17 mm.The glass cube had a flatness of about 0.25 lambda, where lambda equaled632.80 nm (a reference wavelength of light). The roll-laminated MOF wasannealed in an autoclave oven at 60° C. and 550 kPa (80 psi) for twohours. A Zygo Interferometer (available from Zygo Corporation,Middlefield Conn.) was used to measure the flatness of theroll-laminated MOF using light having a wavelength of lambda=632.80 nm.The Zygo Interferometer reported a peak to valley roughness, where atilt correction was used and no sphere correction was applied. The peakto valley roughness measured over the 17 mm×17 mm area was determined tobe 1.475 lambda or about 933 nm.

Example 3 PSA Method Using Vacuum

A piece of the adhesive construction of Example 2 was adhered to a glassprism in a manner similar to that in Example 2. The resulting prism/MOFcomposite was placed into a vacuum chamber equipped with a conventionalvacuum pump. The chamber was evacuated to around 71 cm (28 inches) ofHg, and of the sample held under vacuum for about 15 minutes.

The sample was removed from the vacuum chamber and the roughnessparameters of the MOF were measured as described under “RoughnessMeasurement Method.” and the measured values are reported in thefollowing table.

average stdev Ra (nm) 32 3 Rq (nm) 40 5 Rz (μm) 1.2 0.7

A second prism was attached to the prism/MOF composite using thetechnique and the UV optical adhesive of Example 1. The resultingconfiguration was a polarizing beam splitter.

Example 4

The film of Example 3 was bonded to a transparent glass substrate havinga 7 mm width, a 10 mm length, and a 181 micron thickness. The film wasadhered to the glass substrate using 3M™ Optically Clear Adhesive 8141(available from 3M Company, St. Paul, Minn.). The adhesive thickness was12.5 microns. The glass substrate and film laminate was passed through aroller nip. Next, the laminate was bonded to a substrate at a 45 degreeangle such that the reflected polarization was parallel to thesubstrate, and the transmitted polarization had a nominal incidenceangle of 45 degrees. An MPro 120 picoprojector (also available from 3MCompany) was modified such that light from the illumination source ofthe projector passed straight through the laminate to the LCoS imager ofthe projector with the film side of the laminate facing the LCOS imager,and light selected by the imager was reflected at a 90 degree angle.

Comparative Example C-1

A polarizing beam splitter configuration was created according to U.S.Pat. No. 7,234,816 (Bruzzone et al.). A piece of the adhesiveconstruction of Example 2 was adhered to a glass prism using a handroller thereby forming an MOF/prism composite.

The roughness parameters of the MOF were then measured as describedunder “Roughness Measurement Method” and are reported in the followingtable.

average stdev Ra (nm) 65 20 Rq (nm) 100 18 Rz (μm) 8.6 5.1

A second prism was attached to the prism/MOF composite using thetechnique and the UV optical adhesive of Example 1. The resultingconfiguration was a polarizing beam splitter.

Performance Assessment

The polarizing beam splitters of Example 1, 2, 3 and Comparative ExampleC-1 were assessed for their ability to reflect an image using aresolution test projector. A reference reflector consisting of one ofthe 45° prisms used in the other examples and operating as a totalinternal reflection (TIR) reflector was used to establish the bestpossible performance for the test projector.

A test target with 24× reduction was back illuminated with an arc lamplight source. Attached to the front surface of the test target was a 45°prism, identical to those used in earlier examples (and herein calledthe illumination prism). Light from the test target, travelinghorizontally from the source through the test target, entered one faceof the illumination prism, reflected off of the hypotenuse (via TIR) andexited the second face of the prism. The second face of the prism wasoriented such that the exiting light was directed vertically. Thevarious PBSs from the examples, as well as the reference prism wereplaced on top of the second face of the illumination prism. Thereflecting surface (MOF) in the PBSs as well as the hypotenuse from thereference prism were oriented such that the light reflecting from theMOF or the hypotenuse of the reference prism were directed forward andhorizontal. An F/2.4 projection lens obtained from a 3M™ SCP 712 digitalprojector (available from 3M Company, St. Paul, Minn.) was placed at theexit surface of the PBS or the reference prism and focused back onto thetest target, forming a kind of “periscope” layout.

This optical system was then used to assess the ability of eachdifferent PBS to resolve a test target while operating in a reflectionmode. In the system, an approximately 5 mm×5 mm portion of the testtarget was projected to about 150 cm (60 inches) diagonal. Within thisarea of the test target were multiple repeats of the resolution images.Five different identical repeats of the test target were assessed indifferent locations of the projected image: Top Left, Bottom Left,Center, Top Right and Bottom Right. Each test target was assessed todetermine the highest resolution that was clearly resolved. According tothe protocol, the maximum resolution was required to be resolved as wellas all resolutions below that level. There were instances wherelocalized distortions caused lower resolutions to not be resolved eventhough higher resolutions (in a slightly different location) wereresolved. The reason for this choice is that the full field and not justsmall areas must be resolved in order for the PBS to functioneffectively in a reflective mode.

Multiple samples of each Example were tested. Once the maximumresolutions were established for each location on each PBS, an averageand a standard deviation were computed for each type of prism (that is,for Examples 1 - 3, Comparative Example C-1 and the Reference prism.) An“Effective Resolution” was defined as the average minus two standarddeviations. This metric was determined from the data in “line pairs/mm”(lp/mm) and then expressed in terms of the size of the smallestresolvable pixel which was determined as ½ of the inverse of theEffective Resolution expressed in lp/mm. This definition accounts forthe fact that the resolution is only as good as the minimum resolutionacross the field. The Effective Resolution represents the maximumresolution that the particular PBS set can be expected to reliably(across 95% of the image) resolve.

Table 1 shows the results of the measurements of the different Exampleswithin this disclosure and Table 2 shows the resulting EffectiveResolution. As can be seen, the reference sample can resolve a 5 μmpixel. The PBS from Example 1 can also resolve a very nearly 5 μm pixel.Example 2 is able to resolve down to at least 12 μm and the PBS fromExample 3 can resolve down to 7 μm. All of these constructions should beadequate for at least some reflective imaging applications. On the otherhand, the PBS from Comparative Example C-1 is limited to resolvingaround 18 micron pixels, and would likely not be a robust choice for areflective imaging construction.

TABLE 1 Line Pairs/mm at Five Locations for Samples Top Bottom BottomTop Right Right Center Left Left Example Sample (lp/mm) (lp/mm) (lp/mm)(lp/mm) (lp/mm) Reference A 170.4 170.4 108.0 192.0 170.4 1 B 151.2170.4 120.0 151.2 120.0 1 C 151.2 151.2 108.0 120.0 151.2 1 D 151.2151.2 108.0 134.4 120.0 2 E 151.2 134.4 60.0 108.0 86.4 2 F 134.4 134.467.2 96.0 96.0 2 G 134.4 134.4 96.0 60.0 76.8 3 H 134.4 134.4 96.0 86.4120.0 3 I 134.4 151.2 108.0 96.0 96.0 C-1 J 151.2 134.4 48.0 60.0 76.8C-1 K 120.0 134.4 60.0 96.0 60.0 C-1 L 120.0 120.0 60.0 86.4 86.4 C-1 M134.4 120.0 60.0 60.0 86.4

TABLE 2 Effective Resolution of Exemplary Film Effective EffectiveAverage Std. Dev. Resolution Resolution Example (lp/mm) (lp/mm) (lp/mm)(μm) Reference 162.2 31.7 98.8 5.06 1 137.3 19.6 98.1 5.10 2 104.6 30.942.9 11.65 3 115.7 22.1 71.4 7.00 C-1 93.7 32.8 28.2 17.74

In some cases, the polarizing beam splitter is in the form of a platehaving opposing parallel or near parallel major surfaces. Such beamsplitter plates are thin and have flat outermost and internal majorsurfaces that can lead to high contrast and high resolution imagesprojected onto an image play and/or displayed to a viewer. Thepolarizing beam splitters include a multilayer optical film reflectivepolarizer bonded to one or more thin optically transparent substrates.The transparent substrate may be an inorganic material such as glass, oran organic material such as a polymer, or a combination of an inorganicand organic material.

FIG. 6 is a schematic view of a polarization subsystem 600 that includesa light source 605, a first imager 610 and a polarizing beam splitterplate 620. Light source 605 emits light 625 that illuminates and isreceived by first imager 610. First imager 610 modulates the receivedlight and emits an imaged light 615 that is received by polarizing beamsplitter plate 620. The polarizing beam splitter plate reflects thereceived imaged light as reflected light 695 towards a viewer 680 orscreen 690. Polarizing beam splitter plate 620 includes a firstsubstrate 630, a multilayer optical film reflective polarizer 640disposed on the first substrate, and second substrate 650 disposed onthe multilayer optical film reflective polarizer 640 so that multilayeroptical film reflective polarizer 640 is disposed between first andsecond substrates 630 and 650. Multilayer optical film reflectivepolarizer 640 is bonded or adhered to first and second substrates 630and 650 via respective adhesive layers 660 and 670, where each of thetwo adhesive layers can be or include any adhesive disclosed herein. Forexample, in some cases, one or both adhesive layers 660 and 670 can beor include a pressure sensitive adhesive, a UV cured adhesive, or anoptical epoxy. Polarizing beam splitter plate 620 includes a firstoutermost major surface 622 and an opposing second outermost majorsurface 624 that makes an angle θ with major surface 622, where angle θis less than about 20 degrees, or less than about 15 degrees, or lessthan about 10 degrees, or less than about 7 degrees, or less than about5 degrees, or less than about 3 degrees, or less than about 2 degrees,or less than about 1 degree.

Reflected light 695 propagating toward viewer 680 or screen 690 has aneffective pixel resolution of less than 15 microns, or less than 12microns, or less than 10 microns, or less than 9 microns, or less than 8microns, or less than 7 microns, or less than 6 microns, or less than 5microns, or less than 4 microns. In some cases, polarizing beam splitterplate 620 is thin. In such cases, a maximum separation d between thefirst and second outermost major surfaces 622 and 624 is less than about2 mm, or less than about 1.75 mm, or less than about 1.5 mm, or lessthan about 1.25 mm, or less than about 1 mm, or less than about 0.75 mm,or less than about 0.5 mm. In some cases, first and second outermostmajor surfaces 622 and 624 are planar. In some cases, at least one offirst and second outermost major surfaces 622 and 624 is non-planar. Forexample, in some cases, at least one of first and second outermost majorsurfaces 622 and 624 includes a curved portion, or is concave, orconvex, as generally shown schematically in FIG. 7. In some cases, atleast one of first and second outermost major surfaces 622 and 624curves away from or toward polarizing beam splitter plate 620.

Each of substrates 630 and 650 can be any type substrate that may bedesirable in an application. For example, substrates 630 and 650 caninclude glass or a polymer. Substrates 630 and 650 can each be a singlelayer meaning that there are no embedded or internal major interfaceswithin the substrates. In some cases, at least one of first and secondsubstrates 630 and 650 can include two or more layers. In some cases,substrates 630 and 650 are optically isotropic meaning that thesubstrates have substantially equal indices of refraction along threemutually orthogonal directions. In some cases, substrates 630 and 650have very low light scattering properties. For example, in such cases,each of substrates 630 and 650 has a diffuse transmission of less thanabout 5%, or less than about 4%, or less than about 3%, or less thanabout 2%, or less than about 1%, or less than about 0.5%. As usedherein, diffuse transmission refers to light that is transmitted outsidea 2 degree half-angle cone for collimated normal light incidence.

First imager 605 can be any first imager disclosed herein that may bedesirable in an application. For example, in some cases, first imager605 can include or be an LCOS imager. In some cases, polarizationsubsystem 600 includes a projection lens 675 that receives light frompolarizing beam splitter plate 620 after light is imaged and projects ittowards the viewer or screen as light 695. In some cases, multilayeroptical film reflective polarizer 620 has a surface roughness Ra of lessthan 45 nm or a surface roughness Rq of less than 80 nm, or a surfaceroughness Ra of less than 40 nm or a surface roughness Rq of less than70 nm, or a surface roughness Ra of less than 35 nm or a surfaceroughness Rq of less than 55 nm.

Polarization subsystem 600 can be incorporated into any system that maybe desirable in an application. For example, in some cases, athree-dimensional image projector includes the polarization subsystem600. Light source 605 can be or include any type light source disclosedherein. In some cases, light source 605 includes one or more LEDs. Insome cases, a projection system includes projection subsystem 600 andfirst imager 610 is pixelated and includes a plurality of pixels. Thepixels can form a regular array of pixels forming rows and columns ofpixels. The projection system projects images of the pixels in theplurality of pixels onto a screen. Each pixel has an expected locationon the screen, an expected area on the screen, an actual location on thescreen, and an acctual area on the screen. In some cases, the actuallocation of each pixel on the screen is within a circle that is centeredon the expected location of the pixel and has an actual area that isless than 100 times, or less than 75 times, or less than 50 times, orless than 25 times, or less than 15 times, or less than 10 times, orless than 5 times, or less than 2 times, the expected area of the pixel.In some cases, the actual area of the projected pixel on the screen isless than 10 times, or less than 7 times, or less than 5 times, or lessthan 3 times, or less than 2 times, the expected area of the projectedpixel on the screen.

FIG. 8 is a schematic view of a reflective-type imaging system 800 wherelight 625 emitted by light source 605 is transmitted by polarizing beamsplitter plate 620 toward imager 610 and is reflected by the imager asimaged light 615 toward the splitter plate which reflects the imagedlight as reflected light 695 toward viewer 680. Since multilayer opticalfilm reflective polarizer 640 is substantially flat, the reflectedimaged light 695 has vastly improved effective pixel resolution. FIG. 9is a schematic view of a transmissive-type imaging system 900 wherelight 625 emitted by light source 605 is reflected by polarizing beamsplitter plate 620 toward imager 610 and is reflected by the imager asimaged light 615 toward the beam splitter plate which transmits theimaged light as transmitted light 695 toward screen 690 (or viewer 680similar to system 800 in FIG. 8). Since multilayer optical filmreflective polarizer 640 is substantially flat, light that is reflectedby the beam splitter plate toward the imager illuminates the imager withvastly improved uniformity. FIG. 10 is a schematic view of areflective-transmissive-type imaging system 1000 where imaged light 615emitted by an imaged light source 1005 is reflected by polarizing beamsplitter plate 620 toward viewer 680. Viewer 680 may also view anambient image carried by ambient light 1020 and transmitted by beamsplitter plate 620.

Polarizing beam splitter plate 620 can be manufactured using any processor method disclosed herein. For example, polarizing beam splitter plate620 can be constructed or manufactured using a process disclosed inrelation to FIGS. 4 and 5 except that prisms 560 and 580 are replacedwith substrates 630 and 650.

Polarizing beam splitter plates disclosed herein can be employed in anyapplication where it is desirable to reflect an imaged light from areflective polarizer with no or little degradation in image resolutionand/or contrast. For example, FIG. 11 is a schematic view of a viewingdevice 1100 that provides two different images for viewing by a viewer1101. Viewing device 1100 includes a projector 1110 and a polarizingbeam splitter plate 1130 which can be or include any polarizing beamsplitter plate disclosed herein. Projector 1100 projects a first imagedlight 1120 that propagates toward polarizing beam splitter plate 1130.The polarizing beam splitter plate receives the projected first imagedlight from the projector and reflects the received first imaged light asreflected first imaged light 1125 for viewing by viewer 1101. Thereflected first imaged light has an effective pixel resolution of lessthan 15 microns, or less than 12 microns, or less than 10 microns, orless than 9 microns, or less than 8 microns, or less than 7 microns, orless than 6 microns, or less than 5 microns, or less than 4 microns.Polarizing beam splitter plate 1130 also receives a second image 1132and transmits the second image for viewing by the viewer. Second image1132 can be any type image, such as an ambient image. Polarizing beamsplitter plate 1130 includes a first substrate 1140 similar to firstsubstrate 630 and a multilayer optical film reflective polarizer 1160that is similar to reflective polarizer 640 and is adhered to firstsubstrate 1140 by adhesive 1150. Reflective polarizer 1160 substantiallyreflects polarized light having a first polarization state andsubstantially transmits polarized light having a second polarizationstate opposite to the first polarization state. For example, reflectivepolarizer 1160 reflects at least 70%, or at least 80%, or at least 90%,or at least 95%, or at least 99%, or at least 99.5%, of polarized lighthaving the first polarization state and transmits at least 70%, or atleast 80%, or at least 90%, or at least 95%, or at least 99%, or atleast 99.5%, of polarized light having the second polarization stateopposite to the first polarization state. The second polarization stateis opposite to the first polarization state meaning that neitherpolarization state has a component along the other polarization state.For example, the first polarization state can be a right-handed circularpolarization state and the second polarization state can be aleft-handed circular polarization state. In some cases, the secondpolarization state can be perpendicular to the first polarization state.In some cases, reflective polarizer 1160 can be a broad-band reflectivepolarizer. For example, in some cases, the reflective polarizer cansubstantially reflect polarized light having a first polarization statein a region from 400 nm to 650 nm, and substantially transmit polarizedlight having a second polarization state opposite to the firstpolarization state in the same region. In some cases, reflectivepolarizer 1160 can reflect a first polarization state in two or morediscrete wavelength regions and transmit light in other regions. Forexample, in some cases, reflective polarizer 1160 can substantiallyreflect polarized light having a first polarization state in the 445 nmto 460 nm, 530 nm to 550nm, and 600 nm to 620 nm, wavelength regions,and substantially transmit light of both the first polarization stateand an opposing or perpendicular second polarization state in otherwavelength regions.

Polarizing beam splitter plate 1130 includes a first outermost majorsurface 1165 and an opposing second outermost major surface 1145 thatmakes an angle of less than about 20 degrees, or less than about 15degrees, or less than about 10 degrees, or less than about 7 degrees, orless than about 5 degrees, or less than about 3 degrees, or less thanabout 2 degrees, or less than about 1 degree, with first outermost majorsurface 1165. In cases where one or both major surfaces are curved, theangle between the two major surfaces is determined by finding the anglebetween the best planar fits to the curved major surfaces.

Projector 1110 can be any type projector that may be desirable in anapplication. For example, projector 1110 can include an LCOS imager, anOLED imager, a micro electro mechanical system (MEMS) imager, or adigital micro-mirror device (DMD) imager such as a DLP imager. In somecases, Projector 1110 projects a polarized first imaged light 1120. Insome cases, Projector 1110 projects an unpolarized first imaged light1120. In some cases, first imaged light 1120 can be polarized orunpolarized. In the exemplary viewing device 1100, polarizing beamsplitter plate 1130 receives the projected first imaged light 1120 frommultilayer optical film reflective polarizer 1160 side of the polarizingbeam splitter plate. In some cases, the beam splitter plate can bereversed so that the splitter plate receives the projected first imagedlight from first substrate 1140 side of the polarizing beam splitterplate. In the exemplary viewing device 1100, polarizing beam splitterplate 1130 receives the second image 1132 from first substrate 1140 sideof the polarizing beam splitter plate. In some cases, the beam splitterplate can be turned around so that the splitter plate receives thesecond image from multilayer optical film reflective polarizer 1160 sideof the polarizing beam splitter plate.

In general, polarizing beam splitter plate 1130 can have any shape thatmay be desirable in an application. For example, in some cases, such asshown in FIG. 6 or 8, polarizing beam splitter plate 1130 is flat. Asanother example, in some other cases, such as shown in FIG. 11,polarizing beam splitter plate 1130 is curved. Polarizing beam splitterplate 1130 can be shaped using any method that may desirable in anapplication. For example, the entire plate can be shaped by, forexample, heating the plate. As another example, first substrate 1140 canfirst be shaped and then multilayer optical film reflective polarizercan be applied to the substrate. First substrate 1140 can be any typesubstrate disclosed herein. For example, first substrate 1140 caninclude glass and/or a polymer. In some cases, the maximum thickness offirst substrate 1140 is less than 2 mm, or less than 1.5 mm, or lessthan 1 mm, or less than 0.5 mm, or less than 0.25 mm, or less than 0.15mm, or less than 0.1 mm, or less than 0.05 mm. In some cases, polarizingbean splitter plate 1130 is quite thin. For example, in such cases, amaximum separation between the first and second outermost major surfaces1165 and 1145 is less than about 5 mm, or less than 4 mm, or less than 3mm, or less than 2 mm, or less than 1.5 mm, or less than 1 mm, or lessthan 0.75 mm, or less than 0.5 mm, or less than 0.25 mm.

In some case, the width and length of polarizing beam splitter plate1130 is substantially greater than the thickness of the polarizing beamsplitter plate. For example, in such cases, the ratio of a minimumlateral dimension of the polarizing beam splitter plate to a maximumseparation between the first and second outermost major surfaces of thepolarizing beam splitter plate is greater than 5, or greater than 10, orgreater than 15, or greater than 20, or greater than 30.

The exemplary polarizing beam splitter plate 1130 in FIG. 11 includesone substrate. In some cases, such as shown in FIG. 6, the beam splittercan include two substrates. In such cases, multilayer optical filmreflective polarizer 1160 can be disposed between and adhered to bothsubstrates using adhesives.

FIG. 12 is a schematic view of a head mounted projection display 1200that includes a frame 1210 that is configured to be mounted on aviewer's head. Projection display 1200 further includes a rightprojector 1204 projecting an imaged light and a right polarizing beamsplitter plate 1240 that receives the projected imaged light from theright projector and is attached to the frame such that when the frame ismounted on a viewer's head, the polarizing beam splitter plate faces theviewer's right eye and the projector is disposed on the right side ofthe viewer's head. Projection display 1200 further includes a leftprojector 1205 projecting an imaged light and a left polarizing beamsplitter plate 1245 that receives the projected imaged light from theleft projector and is attached to the frame such that when the frame ismounted on a viewer's head, the polarizing beam splitter plate faces theviewer's left eye and the projector is disposed on the left side of theviewer's head. Head mounted projection display 1200 further includesright and left earpieces 1230 that are attached to the frame such thatwhen the frame is mounted on a viewer's head, the earpieces are disposednear the viewer's ears and are configured to provide audio informationto the viewer. In some cases, projectors 1204 and 1205 and earpieces1230 can be integral to frame 1210.

In the exemplary projection display 1200, the frame is in the form of apair of eye glasses having earpieces 1230 configured to rest on aviewer's ears and a nosepiece 1220 configured to rest on a viewer'snose. In general, the projection display 1200 can have any type framethat may be desirable in an application. For example, in some cases, theframe can be mounted on a viewer's head with the frame surrounding thehead and having means for adjusting the size of the frame. Furthermore,in such cases, the display may have only one polarizing beam splitterand one projector.

FIGS. 13a-i illustrate a method for making or producing an opticalelement. The method first includes a process for making a polymericsheet. First, as shown schematically in FIG. 13a , a bottom substrate1310 is provided. Next, as shown schematically in FIG. 13b , stops 1320are placed on the top surface and along the edges of bottom substrate1310. Next, as shown schematically in FIG. 13c , the cavity defined bystops 1320 is filled with an organic liquid 1330. Next, as shownschematically in FIG. 13d , a top substrate 1342 is placed on stops 1320and organic liquid 1330 and the organic liquid is solidified by dryingand/or curing the organic liquid resulting in a polymeric sheet 1330.The polymeric sheet is then optionally annealed to reduce the opticalbirefringence of the polymeric sheet. In some cases, the maximumthickness h of the polymeric sheet is at least 2 mm, or at least 3 mm,or at least 4 mm, or at least 5 mm.

The method for making or producing an optical element next includes aprocess for making an assembly 1338 shown schematically in FIG. 13e .First, as shown and described elsewhere herein, a temporary flatsubstrate is provided. Next, a first surface of an optical film 1350 isreleasably attached to the temporary flat substrate. Next, a firstpolymeric sheet 1330/1335 is adhered to an opposing second surface ofthe optical film using an adhesive 1340. The optical film is thenremoved from the temporary flat substrate and a second polymeric sheet1330/1337 is adhered to the first surface of the optical film via anadhesive 1345 resulting in assembly 1338 that includes an optical film1350 disposed between and adhered to top and bottom polymeric sheets1335 and 1337 with respective adhesive layers 1340 and 1345, where toppolymeric sheet has a top surface 1335 a and bottom polymeric sheet hasa bottom surface 1337 a. In some cases, the maximum thickness of each oftop and bottom polymeric sheets 1335 and 1337 is at least 2 mm, or atleast 3 mm, or at least 4 mm, or at least 5 mm.

Optical film 1350 can be any type optical film that may be desirable inan application. For example, optical film 1350 can be or include aretarder such as quarter-wave retarder or a half-wave retarder, apolarizer such as an absorbing polarizer or a reflective polarizer. Anysuitable type of reflective polarizer may be used for a reflectivepolarizer 1350 such as, for example, a multilayer optical film (MOF)reflective polarizer, a diffusely reflective polarizing film (DRPF)having a continuous phase and a disperse phase, such as a Vikuiti™Diffuse Reflective Polarizer Film (“DRPF”) available from 3M Company,St. Paul, Minn., a wire grid reflective polarizer described in, forexample, U.S. Pat. No. 6,719,426, or a cholesteric reflective polarizer.

For example, in some cases, reflective polarizer 1350 can be or includean multilayer optical film (MOF) reflective polarizer, formed ofalternating layers of different polymer materials, where one of the setsof alternating layers is formed of a birefringent material, where therefractive indices of the different materials are matched for lightpolarized in one linear polarization state and unmatched for light inthe orthogonal linear polarization state. In such cases, an incidentlight in the matched polarization state is substantially transmittedthrough reflective polarizer 1350 and an incident light in the unmatchedpolarization state is substantially reflected by reflective polarizer1350. In some cases, an MOF reflective polarizer 1350 can include astack of inorganic dielectric layers.

As another example, reflective polarizer 1350 can be or include apartially reflecting layer that has an intermediate on-axis averagereflectance in the pass state. For example, the partially reflectinglayer can have an on-axis average reflectance of at least about 90% forvisible light polarized in a first plane, such as the xy-plane, and anon-axis average reflectance in a range from about 25% to about 90% forvisible light polarized in a second plane, such as the xz-plane,perpendicular to the first plane. Such partially reflecting layers aredescribed in, for example, U.S. Patent Publication No. 2008/064133, thedisclosure of which is incorporated herein in its entirety by reference.

In some cases, reflective polarizer 1350 can be or include a circularreflective polarizer, where light circularly polarized in one sense,which may be the clockwise or counterclockwise sense (also referred toas right or left circular polarization), is preferentially transmittedand light polarized in the opposite sense is preferentially reflected.One type of circular polarizer includes a cholesteric liquid crystalpolarizer.

In some cases, optical film 1350 can be a non-polarizing partialreflector. For example, optical film 1350 can include a partiallyreflective metal and/or dielectric layer. In some cases, optical film1350 can have a structured surface. Optical film 1350 can be organicsuch as polymeric, or inorganic or a combination of the two.

Next, top surface 1335 a of top polymeric sheet 1335 and bottom surface1337 a of bottom polymeric sheet 1337 are modified resulting in a topstructured surface 1335 b having a plurality of top structures 1360 anda top surface roughness 1362 a, and a bottom structured surface 1337 bhaving a plurality of bottom structures 1370 and a bottom surfaceroughness 1362 b. Structures 1360 and 1370 can be any type structuresthat may be desirable in an application. For example, in the case of apolarizing beam splitter, structures 1360 and 1370 can be prisms, suchas right angle prisms. In such cases, each of the plurality of topstructures 1360 and the plurality of bottom structures 1370 includes aplurality of prisms. I some cases, each top prism 1360 and each bottomprism 1370 has a substantially square hypotenuse.

As another example, structures 1360 and 1370 can be lenses as shownschematically in FIG. 15. In some cases, such as in the exemplary caseshown in FIG. 13f , structures 1360 and 1370 are separated by landregions 1362. In some cases, such as in the exemplary case shown in FIG.15, structures 1360 and 1370 are closely packed and are not separated byland regions. Each structure 1360 and 1370 includes a base and aplurality of sides. For example, each exemplary prismatic structure 1360shown in FIGS. 13f and 13g includes a hypotenuse 1360E and four sides1360A, 1360B, 1360C and 1360D. Similarly, each structure 1370 includes ahypotenuse 1370E.

The size or dimensions of surface roughness 1362 a and 1362 b are muchsmaller than the size of structures 1360 and 1370. For example, in somecases, a maximum lateral dimension of hypotenuse 1360E or 1370E isgreater than 1 mm, or greater than 2 mm, or greater than 3 mm, orgreater than 5 mm, or greater than 7 mm, or greater than 10 mm, orgreater than 15 mm, and an average size of top surface roughness 1362 aor bottom surface roughness 1362 b is less than 500 microns, or lessthan 250 microns, or less than 100 microns, or less than 75 microns, orless than 50 microns. In general, top surface roughness 1362 a andbottom surface roughness 1362 b are primarily due toprocessing/manufacturing limitations that lead to unintended deviationfrom optically smooth top structured surface 1335 b and bottomstructured surface 1337 b resulting in surface roughness beingsuperimposed on the intended structures 1360 and 1370.

In some cases, top structured surface 1335 b has a plurality ofregularly arranged top structures 1360 and bottom structured surface1337 b has a plurality of regularly arranged bottom structures 1370.

In general, there is a one to one correspondence between structures 1360and 1370, although, in some cases, there may not be such acorrespondence. For example, in some cases, such as in the exemplaryconstruction shown in FIG. 13f , bottom structures 1370 are verticallyaligned with top structures 1360 and top and bottom land regions 1362are vertically aligned with each other.

In general, any suitable method may be used to modify top surface 1335 aof top polymeric sheet 1335 and bottom surface 1337 a of bottompolymeric sheet 1337 resulting in structured surface 1335 b and 1337 b.Exemplary methods include etching, photolithography, wire EDM, machiningsuch as grinding such as diamond grinding, milling such as diamond endmilling, diamond turning, and fly-cutting such as axial fly-cutting andradial fly-cutting.

The method for making or producing an optical element next includessubdividing assembly 1338 along a lateral direction, such as they-direction, along vertical lines 1341A and 1341B in land regions 1362of the coated assembly into at least two discrete pieces to form adiscrete assembly 1391 shown schematically in FIG. 13h . Any suitablemethod can be used to subdivide assembly 1338. Exemplary methods includecutting such as laser cutting, knife or saw cutting, rotary cutting,razor cutting, die cutting, and ultrasonic vibrational cutting, ormachining such as milling and fly-cutting.

Next, a coated assembly or optical element 1390 is formed by applying atop coating 1380 to top structure 1360 resulting in a top coatedstructure 1367 having a top coated surface roughness 1381 that is lessthan top surface roughness 1362 a, and a bottom coating 1385 to bottomstructure 1370 resulting in a bottom coated structure 1377 having abottom coated surface roughness 1386 that is less than bottom surfaceroughness 1362 b. Any suitable coating method can be to apply top andbottom coatings 1380 and 1385. Exemplary methods include dip coating bydipping the discrete assembly into a coating solution, spray coating,vacuum coating, slide coating, slot coating, curtain coating, andprinting such as inkjet printing. In some cases, such as when thecoating method includes dipping the discrete assembly into a coatingsolution, the discrete assembly is then removed from the coatingsolution and the coating is solidified by drying and/or curing thecoating. In some each of the ratio of top surface roughness 1362 a totop coated surface roughness 1381 and the ratio of bottom surfaceroughness 1362 b to bottom coated surface roughness 1386 is at least 2,or at least 3, or at least 4, or at least 5.

Top and bottom coatings 1380 and 1385 can be any type coating that maybe desirable in an application. For example, in some cases, a primarybenefit of the coatings is to reduce the surface roughness. In somecases, the coatings are hard coatings. In such cases, at least one ofinput and output major surfaces 1396 a and 1396 b has a pencil hardnessof at least 3 H, or at least 4 H, or at least 5 H. In some cases, eachof input and output major surfaces 1396 a and 1396 b has a pencilhardness of at least 3 H, or at least 4 H, or at least 5 H. In somecases, each major surface other than the hypotenuse of each of topstructure 1360 and bottom structure 1370 has a pencil hardness of atleast 3 H, or at least 4 H, or at least 5 H. In some cases, the coatingsare anti-reflective coating with a primary benefit of reducingreflection. In such cases, each of top and bottom coatings 1380 and 1385reduce average reflection in the visible by at least 1%, or by at least2%, or by at least 3%, or by at least 4%. In some cases, the coating canbe a combination of hard coatings and anti-reflective coatings Ingeneral, the coatings can be single layer coatings or multiple coatings.

In some cases, the disclosed method for making or producing opticalelement 1390 can be carried out sequentially. In some cases, the orderof at least some of the steps can be changed. For example, in somecases, the step of forming a coated assembly can be done prior tosubdividing the assembly. As an example, in such cases, top surface 1335a of top polymeric sheet 1335 and bottom surface 1337 a of bottompolymeric sheet 1337 is first modified resulting in a top structuredsurface 1335 b having a plurality of top structures 1360 and a topsurface roughness 1362 a, and a bottom structured surface 1337 b havinga plurality of bottom structures 1370 and a bottom surface roughness1362 b. Next, a coated assembly 1399 is formed by applying a top coatingto the top structured surface resulting in a top coated structuredsurface having a plurality of top coated structures and a top coatedsurface roughness less than the top surface roughness, and a bottomcoating to the bottom structured surface resulting in a bottom coatedstructured surface having a plurality of bottom coated structures and abottom coated surface roughness less than the bottom surface roughness.Next, the coated assembly is subdivided along a lateral direction of thecoated assembly into at least two discrete pieces to form opticalelement 1390.

Optical element 1390 includes an input major surface 1396 a throughwhich an incident light 1383 enters the optical element and an outputmajor surface 1396 b through which the entered light exits the opticalelement as light 1382. Optical element 1390 has low birefringencemeaning that when polarized light 1383 having a first polarization stateenters optical element 1390 from input major surface 1396 a and travelsthrough at least 2 mm of the optical element and exits the opticalelement from output major surface 1396 b, at least 90%, or at least 92%,or at least 95%, or at least 97%, or at least 99%, of light 1382 exitingthe optical element is polarized having the first polarization state. Insome cases, when polarized light 1383 having a first polarization stateenters optical element 1390 from input major surface 1396 a and travelsthrough at least 3 mm of the optical element and exits the opticalelement from output major surface 1396 b, at least 90%, or at least 92%,or at least 95%, or at least 97%, or at least 99%, of light 1382 exitingthe optical element is polarized having the first polarization state. Insome cases, when polarized light 1383 having a first polarization stateenters optical element 1390 from input major surface 1396 a and travelsthrough at least 4 mm of the optical element and exits the opticalelement from output major surface 1396 b, at least 90%, or at least 92%,or at least 95%, or at least 97%, or at least 99%, of light 1382 exitingthe optical element is polarized having the first polarization state. Insome cases, when polarized light 1383 having a first polarization stateenters optical element 1390 from input major surface 1396 a and travelsthrough at least 5 mm of the optical element and exits the opticalelement from output major surface 1396 b, at least 90%, or at least 92%,or at least 95%, or at least 97%, or at least 99%, of light 1382 exitingthe optical element is polarized having the first polarization state.

In some cases, optical element 1390 is a polarizing beam splitter 1390that includes a first polymeric prism 1360, a second polymeric prism1370, and a reflective polarizer 1350 that is disposed between andadhered to a hypotenuse of each of first and second polymeric prisms1360 and 1370 with respective adhesive layers 1340 and 1345. Reflectivepolarizer 1350 substantially reflects polarized light having a firstpolarization state and substantially transmits polarized light having anopposite second polarization state. Polarizing beam splitter 1390further includes a hardcoat 1380/1385 disposed on each of the first andsecond polymeric prisms. Polarizing beam splitter 1390 further includesinput major surface 1396 a and output major surface 1396 b. At least oneof the input and output major surfaces has a pencil hardness of at least3 H, or at least 4 H, or at least 5 H. Polarizing beam splitter 1390 hasa low birefringence such that when polarized light 1383 having apolarization state enters the optical element from input major surface1396 a and travels through at least 2 mm, or at least 3 mm, or at least4 mm, or at least 5 mm, of the polarizing beam splitter and exits thepolarizing beam splitter from output major surface 1396 b, at least 95%,or at least 97%, or at least 99%, or at least 99.5%, of light 1382exiting the polarizing beam splitter is polarized having thepolarization state. In some cases, each of the first and secondpolymeric prisms 1360 and 1370 is a right angle prism. In some cases,first and second polymeric prisms 1360 and 1370 form substantially acube. In some cases, polarizing beam splitter 1390 has a maximumthickness of at least 2 mm, or at least 3 mm, or at least 4 mm, or atleast 5 mm, or at least 7 mm, or at least 10 mm. In some cases, each offirst and second polymeric prisms 1360 and 1370 includespolymethylmethacrylate (PMMA). In some cases, hardcoat 1380/1385includes an acrylate and/or a urethane, or silica and/or a ceramicmaterial. In some cases, the maximum thickness of hardcoat 1380/1385 isless than about 20 microns, or less than about 15 microns, or less thanabout 15 microns, or less than about 10 microns, or less than about 7microns, or less than about 5 microns, or less than about 3 microns, orless than about 2 microns, or less than about 1 micron. In some cases,the difference between the index of refraction of each of first andsecond polymeric prisms 1360 and 1370 and the index of refraction ofhardcoat 1380/1385 is less than about 0.1, or less than about 0.075, orless than about 0.05, or less than about 0.025, or less than about 0.01.In some cases, input major surface 1396 a is parallel to output majorsurface 1396 b. In some cases, input major surface 1396 a isperpendicular to output major surface 1396 b.

EXAMPLE

A PMMA polymeric sheet was provided. The polymeric sheet had a length of44.7 mm, a width of 20.6 mm, and a thickness of 12.5 mm. The top surfaceof the polymeric sheet was diamond fly-cut resulting in a generallyplanar surface having a maximum surface roughness of about 0.5 microns.Next a coating solution was prepared by adding 12.31 grams of 1:1hexanediol diacylate:pentaerythritol triacrylate (HDDA:PETA), 0.51 gramsof acrylamidomethyl cellulose acetate butyrate, 0.21 grams ofbis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide (Irgacure 819), 0.04grams of silicone polyether acrylate (Tegorad 2250), and 29.19 grams ofisopropanol. The solution was stirred at 3500 rpm for 1 minute. Next,the modified polymeric sheet was dipped into the coating solution. Next,the modified sheet was removed, air dried and then baked at 70° C. for 3minutes. Next, the baked sample was cured in an inert atmosphere using36 seconds of 250 mW/cm2 of broad spectrum UV light. The maximum surfaceroughness of the resulting sample was about 0.2 microns.

The present invention should not be considered limited to the particularexamples and embodiments described above, as such embodiments aredescribed in detail to facilitate explanation of various aspects of theinvention. Rather the present invention should be understood to coverall aspects of the invention, including various modifications,equivalent processes, and alternative devices falling within the spiritand scope of the invention as defined by the appended claims.

The invention claimed is:
 1. A polarizing beam splitter comprising areflective polarizer disposed between and adhered to first and secondpolymeric prisms, the reflective polarizer substantially reflectingpolarized light having a first polarization state and substantiallytransmitting polarized light having an opposite second polarizationstate, at least an outer major surface of the polarizing beam splitterhaving a pencil hardness of at least 3 H, such that when a polarizedlight having a polarization state enters the polarizing beam splitterand travels through at least 2 mm of the polarizing beam splitter andexits the polarizing beam splitter, at least 95% of light exiting thepolarizing beam splitter is polarized having the polarization state. 2.The polarizing beam splitter of claim 1, wherein each of the first andsecond polymeric prisms is a right angle prism.
 3. The polarizing beamsplitter of claim 1, wherein the reflective polarizer comprises amultilayer optical film reflective polarizer.
 4. The polarizing beamsplitter of claim 1, wherein when polarized light having a polarizationstate enters the polarizing beam splitter and travels through at least 2mm of the polarizing beam splitter and exits the polarizing beamsplitter, at least 99% of light exiting the polarizing beam splitter ispolarized having the polarization state.
 5. The polarizing beam splitterof claim 1, wherein a difference between an index of refraction of eachof the first and second polymeric prisms is less than about 0.05.
 6. Thepolarizing beam splitter of claim 1 having substantially orthogonalinput and output major surfaces.
 7. The polarizing beam splitter ofclaim 1 having substantially parallel input and output major surfaces.8. A polarizing beam splitter comprising a reflective polarizer disposedbetween and adhered to first and second polymeric prisms, the reflectivepolarizer substantially reflecting polarized light having a firstpolarization state and substantially transmitting polarized light havingan opposite second polarization state, at least an outer major surfaceof the polarizing beam splitter having a first surface roughness andcoated with a coating having a smaller second surface roughness, suchthat when a polarized light having a polarization state enters thepolarizing beam splitter and travels through at least 2 mm of thepolarizing beam splitter and exits the polarizing beam splitter, atleast 95% of light exiting the polarizing beam splitter is polarizedhaving the polarization state.
 9. An optical construction comprising; areflective polarizer substantially reflecting polarized light having afirst polarization state and substantially transmitting polarized lighthaving an opposite second polarization state; a plurality of regularlyarranged top structures disposed on a top major surface of thereflective polarizer; and a plurality of regularly arranged bottomstructures disposed on a bottom major surface of the reflectivepolarizer and vertically aligned with the top structures, at least onestructure in the pluralities of the top and bottom structure having asurface having a first surface roughness and coated with a coatinghaving a smaller second surface roughness.